Why is the synchronous motor current so high during a "sudden" short circuit?
When a synchronous motor experiences a short circuit, the circuit is interrupted, causing the current to lose its normal path and flow only through the short circuit. During a short circuit, the synchronous motor current becomes abnormally high.
A synchronous motor is an AC motor in which the current and magnetic flux are closely related. Under normal operating conditions, the rotor of a synchronous motor rotates synchronously with the magnetic field. The magnetic field generated by the current in the stator windings interacts with the permanent magnets, thereby producing rotational torque. When the rotor is short-circuited, the rotor current suddenly becomes very large.
First, a short circuit causes a decrease in impedance and resistance in the circuit. Normally, the wires in a motor winding are wound on an insulator, and the motor's resistance can be adjusted by changing the type and number of wires in the winding. In a short circuit, the resistance of the short-circuit path approaches zero, causing the current to increase rapidly.
Secondly, a short circuit causes a voltage drop and a decrease in electromotive force (EMF). Under normal operating conditions, the voltage supplied by the power source and the EMF generated inside the motor are balanced, driving the current flow. When a short circuit occurs, the voltage in the circuit is absorbed by the short circuit path, resulting in a very small EMF.
At the same time, the short-circuit path is very short, allowing current to flow easily through it without being limited by the resistance of the motor windings. This further exacerbates the increase in current.
Furthermore, synchronous motors typically continue running when a short circuit occurs because even if the rotor is short-circuited, the stator windings can still generate a magnetic field through the permanent magnets. This causes current to continue flowing along the short-circuit path, thereby increasing the current.
When a motor experiences a short circuit, the rapid increase in current can cause serious damage to the motor and circuitry. The large current can lead to overheating of the motor windings, potentially damaging the winding insulation and even causing the windings to burn out. Therefore, when a motor short circuit occurs, measures should be taken as quickly as possible to cut off the current and prevent further damage.
In summary, when a synchronous motor experiences a short circuit, the circuit is interrupted, and the current loses its normal path, flowing only through the short-circuit path. The reduced impedance and voltage along the short-circuit path, along with the ease with which current can flow through it, cause the current to suddenly become very large. In this situation, measures should be taken to cut off the current to prevent damage.
